Applying standard tools in Thermal Quantum Field Theory, the dissipative coefficient due to the coupling of the inflaton field to a pair of scalars \(\chi _i\)  is given by [1]}, [2]}, [3]}:
\(\Upsilon = \frac{4 g^4 M^2}{T} \int \frac{d^4 p}{(2 \pi )^4} \rho _{\chi }^2 n_B(p_0) ( 1 + n_B(p_0)) \,, \) 
